1. Field of the Invention
This invention relates to a process for the adsorption separation of indole from a mixture of indole with at least one hydrocarbon or a derivative thereof by a zeolite.
2. Description of the Prior Art
Indole is a useful starting material for perfumes, dyes and pharmaceuticals and has recently been spotlighted as a starting material for preparing essential amino acids. Indole is found in coal tar, jasmine oil, neroli oil and putrid protein. Of these substances, coal tar which is produced in a large amount industrially contains 0.2 to 0.5% by weight of indole and a distillate having a boiling point of 220.degree. C. to 270.degree. C. which is obtained by distilling coal tar is said to contain 1.5 to 4.5% by weight of indole.
As the known process for separating indole from coal tar, firstly the acid component and the base component are removed from the distillate having a boiling point of 220.degree. C. to 270.degree. C. obtained by distilling coal tar and secondly the remaining oily substance is heated with sodium or sodium amide at a temperature of 100.degree. C. to 125.degree. C. or with potassium hydroxide at 200.degree. C. to 250.degree. C. to isolate indole sodium or indole potassium. This process requires not only plural steps but also expensive sodium or sodium amide and moreover, the procedures are complicated. As a result, this process can hardly be said to be efficient. In another process, the distillate from which the acid component and the base component have been removed is repeatedly subjected to fractional crystallization at a temperature of -20.degree. C. to +25.degree. C. to remove mainly naphthalene and 2-methylnaphthalene, and its mother liquor is subjected to rectification to separate crude indole, followed by recrystallizing the crude indole from petroleum ether. This process requires plural steps and rectification of high boiling point substances and thus, a large amount of energy is disadvantageously necessary. Also Koks Khim 4, 34 to 37 (1978) describes the separation of indole from a distillate of coal tar containing indole by gas chromatography. This process which does not require complicated procedures is excellent in simply obtaining indole but the starting material must be gasified and also the temperature of the column must be maintained at a temperature of 200.degree. C. to 300.degree. C. As the disadvantages of this process, not only a large amount of energy is necessary but also the apparatus becomes remarkably large in handling gaseous substances. Thus, this process is not suitable for handling a large amount of the starting materials. Thus, the conventional process for separating indole from coal tar is troublesome and not efficient.
It has now been found that in separating indole from its mixture with at least one hydrocarbon or a derivative thereof, indole can be easily and efficiently separated by contacting the mixture with a faujasite structured zeolite. More specifically, when the mixture as such or a solution of the mixture dissolved in an appropriate solvent other than a desorbent A as defined below is contacted to the zeolite, each substance of the mixture is adsorbed on the zeolite but the adsorbability to the zeolite is different among indole and substances other than indole and as a result, the composition of the zeolite phase becomes rich in indole. Thus indole can be separated and recovered by replacing the indole in the zeolite phase by an appropriate desorbent.